Bookmap metadata elements

The Bookmap specialization of ditamap supports standard
book production for collections of DITA topics. This section contains
the metadata elements used by bookmap to store book-related metadata.

Note: The OASIS document type for the bookmap specialization includes the xNAL domain specialization
for describing the author or authors of a document. All elements that appear in that domain (the
authorinformation element and all of its descendants) are described in
the xNAL domain section, rather than in this bookmap metadata section.